best oversized extrusions from Robotdigg?

Right now I have 20x20 extrusions in my 1meter tall/300mm bed delta with metal Robotdigg corners. Even with an all-metal frame the twist factor is not very stiff.
When I made this machine they were not available but now I see several metal-corner options for thicker towers. The question is...which is better for me?
The 30x30s seem like they would be the best: 900 cm^2 cross-section vs 800 cm^2 cross-section of the 20x40 extrusions. They are also more symmetric to loads.
On the other hand, the 20x40s would fit my current wheeled carriages I custom machined and don't want to waste the effort. 20mm is also a more standard dimension for future proofing.
But as accurate as my home-made carriages are, they are heavy and one day I should switch to recirculating-ball rails anyway. My delta is almost completely free of printed parts (the endstop switches still use printed mounting blocks, oh my!)

Any experience or thoughts you can add to my conclusions would be helpful.
thanks

My gut feeling tells me the 2040 extrusion will do. While you build the frame, the 20mm side is strong enough to withstand any twisting ( if any ).
During printing the 40mm side offers more strength than 3030 profile. ( No twisting forces then )

If I were in your shoes, I'd simply add linear rails to the existing 2020 frame and see if it's already done the trick.

I've got a 2040 frame same size 1m x 330mm with robotdigg cast aluminium corners. It's stiff enough for me. But you could make your 2020 frame stiff enough with cross bracing especially using turnbuckles and threaded rods or something like that. A lot of structures achieve incredible strength and stiffness with this approach.

That being said I like the idea of precision drilled end plates and 2080 cbeams used on the Tevo LM. It's very clean and simple, leaving a lot of space for fitting whatever you need, just drill and tap wherever you want. If the end plates were a stock item and easy to get it would make a large, versatile, stiff frame quite affordable to achieve.